Culturing mammalian cells, especially human cells, is crucial in life science, pharmaceutical, and biotechnology research, as human cell cultures may be used to determine cell function and interactions and, for example, produce therapeutic cells, large quantities of proteins, and pathogens for the development of vaccines. Human gene products are commonly produced utilizing in vitro methods via bacterial, yeast, and insect gene expression systems. However, normal biological functionalities, activities, and metabolism of many human proteins are dependent on post-translational modification, such as glycosylation, ubiquitination, proteolytic cleavage, and disulfide bridges. These post-translational modifications can only be properly carried out in mammalian and sometimes only in human cells. In addition, proteins with a molecular weight above 30,000 Daltons (Da) generally cannot be expressed by micro-organisms. To produce such human proteins with their normal biological functions, mammalian cell expression systems may be the only suitable systems. Therefore, the ability to scale up the production of these proteins and the costs of such production are dependent on the yields of mammalian cell cultures, which are often proportional to the cell density of a cell culture.
Culturing anchorage-dependent mammalian cells in a three-dimensional (3-D) suspension model is one way of achieving high density culturing of cells. To achieve higher cell density growth, organotypic culture and microcarrier methods are used as the two major improved culture models. Although organotypic culture is real 3-D growth, the achievement of high density culture is still impeded by nutrient diffusion. The microcarrier method provides an anchor surface and allows culture of anchorage-dependent cells in a suspension model. However, cells typically grow as monolayers on the surface of the microcarriers suspended in the culture medium by gentle agitation, which is not true 3-D growth.
Most mammalian cells derived from solid tissues can only grow in an adherent mode. Affected by gravity, anchorage-dependent mammalian cells typically grow as monolayer on the lowest surface of the culture container. Since it is difficult to achieve high cell density growth in the conventional adherent growth mode, the cost of commercially produced pharmaceutical products by mammalian cell culture is very high. In addition, cells growing in a monolayer may be susceptible to damage resulting from collision of the cells during agitation of the culture medium or a bioreactor. In addition, such monolayer growth systems may be vulnerable to contamination, since they are directly exposed to the culture environment.
Furthermore, the surfaces of conventional cell growth microcarriers are often treated (e.g., by coating the carriers with collagen, fibronectin, laminin and Poly 1-lysine) to promote and enhance attachment of the cultured cells. The use of such coating materials and coating procedures significantly increases the cost of cell carriers and their related products.
Water soluble cationic copolymers presently have very limited application in the biomedical field due to their known high cytotoxicity. The cytotoxicity is known to result from the high charge density of soluble and mobile molecules in the solution.
Accordingly, systems for growing mammalian cells that overcome at least the above-discussed disadvantages are needed.